For information communication and processing systems to operate reliably, it is important to be able to test these systems and measure various performance characteristics that pertain to them. Classically, it has been very difficult to observe the fidelity of a signaling system from a transmit circuit to a receive circuit, including a medium through which the transmit circuit is coupled to the receive circuit. It has been especially difficult to obtain in-situ measurements of the operation of the system. Rather, external test equipment is typically introduced into the system for the purpose of obtaining measurements. It is common for an external signal generator, for example one capable of producing test signals with ultrafast or adjustable transition times, and an external measurement device, such as an oscilloscope, to be connected to a system under test. However, since such external test equipment has characteristics different from system under test, measurements derived using the external test equipment may not accurately reflect the actual performance of the system under test.
While it was possible to obtain meaningful information from simpler systems of the past using external test equipment, the increasing complexity and operating frequencies of modern systems introduce additional complications that impair the effectiveness of testing using external test equipment. For example, much higher frequencies and controlled impedances make it much harder to introduce external test equipment without distorting the signals being measured and, therefore, affecting the measurements themselves. Moreover, connection and disconnection of the external test equipment requires time, effort, and, potentially, additional design considerations, such as the provision of test points within a system. Also, external test equipment does not allow testing to be performed from the perspective of the actual receive circuit within the system. Thus, such testing cannot definitively provide information as to what the receive circuit actually receives. Therefore, traditional testing techniques fail to provide complete and accurate information about the system under test. Thus, a technique is needed to provide complete and accurate information about a system under test and to enable in-situ testing of the system.